I think the particular reason that AF, BA and LH all have lower requirements for members from outside the home country, is that pax have to connect to get anywhere.

For example: someone living in Paris can usually fly direct on AF, or connect at LHR on BA. Since most high yield J pax would just pick the direct flight, BA needs to offer more to draw that revenue -- hence, lower fares and lower status requirements.

I think this is less true of AC in the UK, who offer more service and better frequencies to Canadian cities than BA, and given the saturation of the transatlantic market, are unlikely to become a major transfer airline for UK-US pax.